1. Field of the Invention
The present invention relates to a process for removing scales adhered on equipment, especially condenser surfaces, which are in contact with the vapor produced by the evaporation of a digested mixture of lignocellulose containing anthraquinone during the condensation of a black liquor obtained by separating pulp from the digested mixture.
2. Description of the Prior Art
It is known that anthraquinone (hereinafter referred to as AQ for 9,10-anthraquinone) and anthraquinone precursors such as anthrahydroquinone (referred to as AHQ), 1,4,4a,9a-tetrahydroanthraquinone (referred to as THAQ) and 1,4-dihydroanthrahydroquinone (referred to as DDA) are excellent for promoting digestion (cooking or pulping) of lignocellulosic materials, such as wood.
When an AQ or AQ precursor is used as a digesting assistant (cooking additive or pulping additive) for lignocelluloses in the digestion of pulp, 40 to 50 wt.% of the AQ or AQ precursor added remains in the form of AQ or AHQ as a reduced product in the black liquor depending upon the kind of the digesting assistant. The black liquor is usually oxidized and concentrated in a multiple effect evaporator and the concentrated liquid is then fired to burn organic materials and to recover inorganic compounds such as sodium carbonate and sodium sulfide in the case of the kraft process. The inorganic compounds are converted into sodium hydroxide and the product is reused.
In order to improve heat efficiency, steam evaporated at higher temperature is fed into another evaporator at lower temperature containing the black liquor of lower concentration and is used for heating in the next evaporator in the multiple effect evaporator for concentrating the black liquor. In the process, the AQ component in the black liquor is evaporated, together with steam, and fed into a steam heating part of an evaporator at lower temperature in which the steam is condensed to be discharged as a drain. Thus, the AQ component has lower solubility and accordingly, it adheres on a steam heat transferring surface of the evaporator, a pump for discharging, a pipe or another heat exchanger to form a scale whereby the heat exchange efficiency is reduced and the equipment may be clogged.
There has now been provided a simple process for removing the adhered scale with industrial advantages. The composition of the scale has been analyzed and found to contain 20 to 90 wt.% AQ component, 10 to 20 wt.% volatile oily component such as an abietic acid ester derived from lignocellulose and the remainder being inorganic salts.
Since the oily component in the adhered scale imparts an effect as a binder to the scale, the scale is heat-treated in an aqueous solution of sodium hydroxide to hydrolyze the oily component and a reducing agent, such as sodium hydrosulfite is used to dissolve the anthraquinone component. As a result, the oil component as the binder is hydrolyzed to dissolution and the scale is dislodged and the AQ component is easily reduced to dissolve into the aqueous solution and the scale is then easily removed.